1. Field of the Invention
This invention relates to target acquisition and tracking and more specifically to the use of a generalized Hough transform to detect vehicles in an image to improve target acquisition, tracking and aim point selection by air and ground launched missiles.
2. Description of the Related Art
Precision image guided air-to-surface missiles are widely used against vehicular targets. These missiles include a seeker located near the nose of the missile to guide the missile to the intended target. A seeker consists of an imager and a tracker to perform the guidance function. The imager, typically infrared, located near the nose of the missile creates an image of the scene. The images are presented to the tracker which locates the position of the target in the image continuously. The tracker's estimated position of the target in the image is used to guide the missile to impact the target. The process of finding the target position in the image is called “tracking”. Central to the imaging tracker is the track gate, which encloses the target to be tracked and delineates that portion of the image as target-occupied.
Target position is derived from the position of persistent features within the track gate. When the track gate is too large, then non-target elements, called background clutter, are also included in the track gate. The features that belong to background will perturb the track point away from the desired point on the target. When the target moves differently from the background clutter in the track gate, the background in the track gate can move the track gate and the track point off of the target and cause the missile to miss the target. This is called a “breaklock”. To minimize breaklocks, it is essential that the track gate be as conformal to the target as possible to exclude background clutter. However, a number of conditions have militated against the maintenance of a conformal gate. These conditions often lead to catastrophic breaklocks or cause the track point to impact an undesirable portion of the target.
Breaklock and other related problems have several causes in different guided missile systems including: gate handoff from the automatic target recognizer (ATR) to the missile tracker, changing aspect ratio of maneuvering targets, limited lock-on after launch (LOAL), aimpoint designation and fire control system to missile tracker handover.
In many emerging military applications, targets are found by automatic target recognizers. These ATRs estimate the position and size of the target in the image and handoff an initial track gate to the missile tracker. However, these gates are often not sufficiently conformal to the target and include a large amount of background clutter in the track gate. Simulation studies have shown that the probability of tracking the target with these poorly defined gates can be degraded by a factor of two relative to that achieved with a well defined gate.
When a target turns its aspect ratio changes. To maintain conformance with the target, the gate must adapt to the changing silhouette. Some older forms of trackers, the gated video tracker, were able to make the needed adaptation by segmenting out the target from the background in situations where there are large contrast difference between the target and the background. However, in cases where contrast is small or when the background is inhomogeneous, these trackers have a tendency to expand the gate to include appreciable background and cause a breaklock. In other cases where the target is inhomogeneous, these gated video trackers often collapse the gate to a locally high contrast region, for example a hotspot, and result in guiding the missile to a less vulnerable portion of the target. A separate class of imaging trackers circumvents the problems of gate adaptation to background and gate collapse by maintaining a fixed gate aspect ratio and growing the gate solely on the basis of estimated range to target. Because the track gate's aspect ratio is fixed, the gate will not be conformal to the target boundary when it turns. If the apparent target silhouette shrinks due to turning, then the track gate will be too large and will include background clutter that may induce a breaklock.
In network centric warfare the observer and shooter are physically separated. In this environment weapons need to have a post launch automatic target acquisition capability. For precision guidance against moving targets, there are two approaches that are currently under consideration. Approach 1 utilizes an ATR to scan the area and find a target. The problems with this approach are: (1) ATR technology is still in the development stage and its viability is not certain; (2) ATRs need expensive high resolution imagery and thus are not compatible with near term retrofit of legacy weapons systems. Approach 2, uses a laser designator and a laser receiver onboard the missile. The designator aims a laser beam at the target and the laser receiver homes in on the scattered laser beam. This is a well known technology but requires the designator to continuously designate the target until missile impact. The prolonged designation time renders the designator susceptible to detection by the enemy's threat warning receiver and significantly increases his mortality rate.
Against heavily armored targets such as main battle tanks, aimpoint selection is critical. It is not sufficient just to have the missile hit the target, the missile must impact the target in a location where the armor is relatively thin. Typically, this spot is not at the centroid of the target. Some current missiles home in on the centroid and use a large warhead to overwhelm the target. This approach is only applicable to large missiles that are carried by large platforms such as aircraft. Smaller, man-portable missiles are not effective. Other postulated approaches involve the use of ATRs. These approaches incur additional costs (for the ATR). Another current approach essentially assumes the track gate defines a rough boundary of the target and attempts to bias the aimpoint away from the center of the gate using prior knowledge of the target shape. This approach suffers from not knowing the aspect of the target and has generally resulted in large aimpoint dispersion.
During the target acquisition process, the target is often identified in a high resolution forward looking infrared (FLIR) either manually or with an ATR. Once the target is identified in the FLIR, the target information needs to be handed over to the missile to initialize the tracker. In the handover process, one needs to determine the relative lines of sight of the FLIR and the missile seeker. The state of the art for determination of the relative lines of sight is to use a boresight correlator. Boresight correlation is performed by first resampling the FLIR image to a coarser resolution to match the seeker's resolution. Next, the missile image is sent across a data bus and correlated against the re-sampled FLIR image. The correlation process is well known to those skilled in the art. The position of the correlation peak indicates the position of the center of the missile image relative to the FLIR's boresight (center of the FLIR's field of view). Once this relative boresight offset is known, the target position and size from the FLIR can then be mapped into the missile image. The problem with this approach is that a large amount of data (missile image) must be sent across the data bus. In many current systems, the data bus does not support this amount of data transfer.